Adding values in columns from 2 dataframes

Question

I have 2 dataframes as below, some of the index values could be common between the two and I would like to add the values across the two if same index is present. The output should have all the index values present (from 1 & 2) and their cumulative values.

    Build   
2.1.3.13    2   
2.1.3.1     1   
2.1.3.15    1   
2.1.3.20    1   
2.1.3.8     1   
2.1.3.9     1   
    
Ref_Build       
 2.1.3.13   2   
 2.1.3.10   1   
 2.1.3.14   1   
 2.1.3.17   1   
 2.1.3.18   1   
 2.1.3.22   1   

For example in the above case 2.1.3.13 should show 4 and the remaining 11 of them with 1 each.

What's the efficient way to do this? I tried merge etc., but some of those options were giving me 'intersection' and not 'union'.

Your help is much appreciated.

Answer 1

You can try merge with outer option or concat on columns

out = pd.merge(df1, df2, left_index=True, right_index=True, how='outer').fillna(0)
# or
out = pd.concat([df1, df2], axis=1).fillna(0)

out['sum'] = out['Build'] + out['Ref_Build']

# or with `eval` in one line
out = pd.concat([df1, df2], axis=1).fillna(0).eval('sum = Build + Ref_Build')

print(out)

          Build  Ref_Build  sum
2.1.3.13    2.0        2.0  4.0
2.1.3.1     1.0        0.0  1.0
2.1.3.15    1.0        0.0  1.0
2.1.3.20    1.0        0.0  1.0
2.1.3.8     1.0        0.0  1.0
2.1.3.9     1.0        0.0  1.0
2.1.3.10    0.0        1.0  1.0
2.1.3.14    0.0        1.0  1.0
2.1.3.17    0.0        1.0  1.0
2.1.3.18    0.0        1.0  1.0
2.1.3.22    0.0        1.0  1.0